The present invention relates to the field of electronic blood pressure measuring devices, and in particular relates to an electronic blood pressure measuring device which measures blood pressure by detecting the amplitude of a pulse wave and to an electronic blood pressure measuring device which determines blood pressure according to smoothed pulse wave information.
According to a prior art type of electronic blood pressure measuring device based upon the so called oscillation method, a cuff is wound on the upper arm of the person whose blood pressure is desired to be measured, such as a patient, and, after the cuff is pressurized to a certain pressure level, wave form information is detected from the cuff pressure and a pulse wave component contained in the cuff pressure during the depressurization of the cuff is isolated, so that the average blood pressure, the maximum blood pressure, and the minimum blood pressure of the person whose blood pressure is being measured may be determined from such data, by finding the amplitude of the pulse wave for each heart beat, and determining the blood pressure from the variation curve (envelope line) of the pulse wave amplitude values and the cuff pressure.
In such an electronic blood pressure measuring device, conventionally the cuff pressure associated with each interval is determined as the cuff pressure at the beginning or at the end of the corresponding time interval. However, according to such a conventional electronic blood pressure measuring device, since the cuff pressure corresponding to each time interval is read out from the cuff pressure at the beginning or at the end of the corresponding time interval, the time points at which the pulse wave appears as its maximum value and minimum value in each interval are different from the time point at which the cuff pressure is read, and therefore some error has in the prior art existed between the parameter (maximum level difference) and the cuff pressure, thereby reducing the accuracy of blood pressure determination provided by the device.
Further, since the pressure value corresponding to the parameter is either at the beginning or at the end of the time interval of the cuff pressure signal in which the pulse wave component is combined and therefore the beginning (or the end) of the interval may correspond either to the maximum point of the pulse wave or to the minimum point of the pulse wave depending upon the particular instance, the curve of the cuff pressure signal tends to oscillate within the amplitude of the pulse wave component, whereby some fluctuations exist in the cuff pressure value, and some distortion exists in the curve of the parameter, thereby reducing the accuracy.
Considering another aspect of the present invention, there is a conventionally known blood pressure measuring technology, known as the Riva-Rocci-Korotkoff method or the auditory method, which has been for some time known as a blood investigation method. According to an electronic blood pressure measuring device based upon this Riva-Rocci-Korotkoff method, after a cuff is wound around the arm of a patient and the cuff is pressurized for stopping blood flow, as the pressure is reduced gradually, the blood starts flowing and a certain distinctive blood sound (the so called Korotkoff sound) is produced, which is detected by a microphone or the like, and then subsequently this sound diminishes as further depressurization of the cuff progressively takes place. The cuff pressure at which the Korotkoff sound is started is then determined as being the maximum blood pressure of the patient, and the cuff pressure at which the Korotkoff sound disappears is determined as being the minimum blood pressure of the patient, in determining the blood pressure of the patient.
As another blood pressure measuring technology, inserting a cannulae into the artery of a patient is known as a blood investigative method.
However, according to an electronic blood pressure measuring device based upon the Riva-Rocci-Korotkoff method among such auditory type conventional blood pressure measuring technologies, the obtained Korotkoff sound is a very small amplitude type signal and its frequency range is from 30 Hz to 150 Hz. Thus, there has been a problem that, since this frequency range tends to be affected by external noises and oscillation noises, especially external and oscillation noises containing the same frequency range or similar frequency ranges, these noises could become a cause of erroneous detection, and such effects have often caused errors in blood pressure measurements in prior art devices for blood pressure measurement by the auditory method. Also, it is hard to recognize any Korotkoff sound from an infant or an adult who is in a state of shock, and in such cases measurement of blood pressure by the auditory method is sometimes impossible. Further, according to such an electronic blood pressure meter based upon the auditory method, since a sensor (such as a microphone) for detecting the Korotkoff sound and also a drive circuit therefor are necessary, therefore the cost of the blood pressure meter tends to be high.
And on the other hand, according to a blood pressure measurement based upon a direct method such as introducing a cannulae into a blood vessel of the patient, the pressure of an artery is transmitted to an external blood pressure transducer by way of a cannulae filled with physiological saline, and in such a method the length of the cannulae, mixing of bubbles therein, and zero point drifts of the blood pressure transducers could cause errors in blood pressure measurement. These errors can be reduced by proper handling, but such handling requires skill and care, thus requiring certain hard to provide techniques in carrying out proper blood pressure measurements. Furthermore, such direct methods as described above have the serious disadvantages that such invasive procedures inevitably cause pain, discomfort, and mental strain to the patient, and increase the possibility of blood tube pain and infections.
Further, according to a conventional type of electronic blood pressure meter based upon the oscillation method, the amplitude of the pulse wave corresponding to the changes in the cuff pressure must be detected, and in order to detect the pulse wave amplitude for each heart beat it is necessary to separate the pulsation on the pulse wave signal which is continuous for each pulse beat. This recognition and separation of pulsations are conducted by feeding the pulse wave signal into a CPU of a microcomputer or the like as pulse wave data and executing a program in the CPU. Therefore, because complicated arithmetic processing is required, a microcomputer having a memory (ROM) of relatively large capacity is necessary, and furthermore the development of the program therefor is difficult, thereby increasing the cost of the electronic blood pressure meter. Also, if the patient has an irregular pulse, or if his or her pulse is relatively weak, the recognition of the pulse beat may not be properly performed, and sometimes accurate measurement of blood pressure becomes impossible. Further, if the patient moves his or her arm during measurement, accurate measurement of blood pressure becomes impossible, because the recognition of the pulse beat is not performed properly, or because the pulse wave amplitude cannot be detected accurately.